1. Field of the Disclosure
This disclosure relates to a PON system.
2. Discussion of the Background Art
As broadband becomes widely available, access network communication is playing an increasingly important role, and thus, an access network is required to supply a service stably.
FIG. 23 shows a configuration including PON (Passive Optical Network) 11 and L2SW (Layer 2 switch) 12 as a typical configuration of a current access network. The PON 11 is constituted of an Optical Line Terminal (OLT, station-side termination apparatus) 14 storing a plurality of OSUs (Optical Subscriber Units) 15, an M:1 optical splitter 18, and an Optical Network Unit (ONU, subscriber termination apparatus) 17, and a plurality of (for example, 32) ONUs 17 are connected to an OSU 15. In the PON 11, since the OSU 15 and an optical fiber 19 between the OSU 15 and the optical splitter 18 are shared by M ONUs 17, the service can be provided at low cost. The L2SW 12 is provided with a large number of ports connected to the PON 11 and a core network 13, and each port on the PON 11 side is connected to one OSU 15 through a line. Since the L2SW 12 is provided with a line concentration function, lines of a number smaller than the number of the lines connected to the port on the PON 11 side are connected to the port on the core network 13 side, and lines from the OSU 15 are concentrated to be connected to the core network 13. When the L2SW 12 receives a down frame from a high-order section, the L2SW 12 reads an identifier (for example, VID (Virtual LAN Identifier)) attached to the frame. Since association of a VID number and an ONU number and corresponding information of the number of a connection destination OSU of each ONU are previously registered in the L2SW 12, the connection destination OSU number of the ONU associated with the corresponding VID number is found from the corresponding information and a received VID number. Since corresponding information between a PON side port number of itself and the connection destination OSU number is also registered in the L2SW 12, a received down frame is output from the corresponding PON side port to transmit the down frame to a desired OSU 15 in accordance with the information. Meanwhile, regarding an up frame from each OSU 15, after the L2SW 12 receives the up frames, the up frames are output from the L2SW 12 to a high-order section in accordance with a transmission order determined by an arbitrary scheduling method. In an example shown in FIG. 23, there is a one-to-one relationship in connection between the port on the PON 11 side of the L2SW 12 and the OSU 15, and since the L2SW 12 is provided with the ports on the PON 11 side having a number that can store a plurality of the OLTs 14, for example, when the maximum number of the OLTs 14 are stored, lines having the same number as the number of all the OSUs 15 stored in the all the OLTs 14 are connected between the L2SW 12 and the OLT 14.
As described above, since the L2SW 12 and the OLT 14 store a large number of users, if service interruption occurs due to failures of the devices, a large number of users are affected thereby. Accordingly, in order to supply service stably, it is important to make devices and paths redundant in preparation for device failure and fiber interruption. Although the L2SW 12 having a larger storable number is relatively often made redundant, it is considered that the importance of making the PON 11 redundant is enhanced, considering future diversification of services and an increase of the storage number in the OLT 14.
In the above description, although the L2SW 12 is externally attached to the OLT 14, the L2SW 12 may be built in the OLT 14. Further, a plurality of the OLTs 14 may share the L2SW 12. (The configuration of FIG. 23 shows that the L2SW 12 is externally attached to the OLT, and a plurality of the OLTs 14 share the L2SW 12.) Furthermore, the L2SW 12 may have a multistage configuration.
To make a portion of or all devices and fibers constituting the PON 11 redundant is referred to as PON protection, and there are various methods corresponding to a redundancy portion and a redundancy method. For example, a method in which the correspondence between a normal system and a redundancy system is N:1, namely a redundancy system (backup) device is provided with respect to N existing (normal system) devices is referred to as an N:1 protection, and the method is proposed in Non-patent Literature 1. The configuration of the N:1 protection is shown in FIG. 24. If the OSU can store up to A ONUs 17, this method uses one OLT 14 storing N normal system OSUs and the redundancy system OSU, A×N or less ONUs 17, and an N×(N+1) optical switch 16 (which is an optical switch in which an arbitrary input/output port of N input/output ports (N side) and an arbitrary input/output port of N+1 input/output ports ((N+1) side) are connected, and there may be an unconnected port). The ONU 17 is connected to each of the N input/output ports on the N side of the optical switch 16 through a PON line 19. The OSU 15 is connected to each of the input/output ports on the (N+1) side of the optical switch 16. Among the OSUs 15, N OSUs are normal systems, and one OSU is a redundancy system. In a normal time, each of the N normal system OSUs 15 communicates with the ONU 17 subordinate to the OSU 15 through the optical switch 16 and each of the N PON lines 19. Meanwhile, since the redundancy system OSU 15 does not establish a path to any of the PON lines through the optical switch 16 in a normal time, the redundancy system OSU 15 does not provide transmission of data to any of the ONUs 17.
When one of the N normal system OSUs 15 is failed, the optical switch 16 switches a path, and a transmission path between the PON line 19 connected to the failed normal system OSU 15 and the redundancy system OSU 15 is established. The ONU 17 in the PON 11 performs ONU registration by the OSU 15 and measures a signal propagation time between the OSU 15 and each ONU 17 before start of data communication, and a link should be established between the OSU 15 and the ONU 17. For example, in a GE-PON (Gigabit Ethernet (registered trademark) PON) system described in Non-Patent Literature 2, an MPCP (Multi Point Control Protocol) frame are transmitted and received between the OSU 15 and the ONU 17 based on a provision of MPCP, whereby a registration/signal propagation time is measured, and, an MPCP link is established between the OSU 15 and the ONU 17. Accordingly, those processes are first required in GE-PON. After the establishment of the MPCP link, the ONU 17 establishes an OAM (Operation, Administration and Maintenance) link with the OSU 15 and exchanges an authentication/encryption key with the OSU 15 through transmission and reception of an OAM frame as a maintenance monitoring signal, and only when all the processes are finished, data communication can be started.
However, since the redundancy system OSU 15 in the system of FIG. 24 does not grasp information of, for example, registration and authentication associated with the ONU 17 subordinate to the failed normal system OSU 15 before switching, the link between the failed normal system OSU 15 and the ONU 17 subordinate to the OSU 15 is cut by the failure of the normal system OSU and path switching performed by the optical switch 16. In the PON system, since data transmission can be started only by establishment of the link and authentication/encryption key exchange between the ONU 17 and the OSU 15, the ONU 17 subordinate to the normal system OSU 15 failed after the path switching establishes the link with the redundancy system OSU 15 and thereafter starts the data transmission.
As described above, even if the normal system OSU 15 is failed, the subordinated ONU 17 can start communication with the redundancy system OSU 15 through the optical switch 16, and therefore, even if the OSU failure occurs, the communication can be recovered by switching in comparison with the case without protection. Further, as a feature, in the N:1 protection, since the cost of the one redundancy system OSU 15 can be shared by all users subordinate to the N normal system OSUs 15, as the value of N is increased to construct the system, additional cost accompanying the protection per the user can be reduced.
As described above, the PON protection is a method for enhancing reliability, and when a normal system device is failed, the normal system device is switched to a redundancy system, whereby communication interruption can be prevented. However, when such switching is performed not only when such a failure occurs but also in a normal time, FW update suppressing the communication interruption of the ONU 17 can be realized. Namely, an arbitrary switching source OSU 15 (for example, the normal system OSU) is switched to a switching destination OSU 15 (for example, the redundancy system OSU) by operation control in a normal time, communication of the ONU 17 subordinate to the corresponding OSU 15 is maintained through the redundancy system OSU 15, and meanwhile, when FW of the switching source OSU 15 is updated, ONU communication interruption corresponding to at least a restart time of the OSU 15 can be suppressed in comparison with the FW update in a system free from the PON protection.